Method and apparatus for cold rolling mill gauge deviation correction

ABSTRACT

Mill spring constant change signals are produced after each of a series of screwdown regulation cycles where each change signal is produced as a function of the ratio of the gauge deviation remaining after the regulation cycle, to the change in rolling force which occurred during the regulation cycle. The series of ratio change signals are then averaged to produce a mill spring constant correction signal which is used to correct the previously established mill spring constant. The corrected mill spring constant is then used to generate a new screwdown reference signal for the next series of regulation cycles. Provision is made to limit check and average gauge deviation so that the mill spring corrections are only made when certain predetermined gauge deviation limits are exceeded.

United States Patent 1 1 Danehy et al.

[ Dec. 11, 1973 METHOD AND APPARATUS FOR COLD ROLLING MILL GAUGEDEVIATION CORRECTION Primary Examiner-Milton S. Mehr Att0rneyF. l-l.Henson et al.

[75] Inventors: Robert P. Danehy; Bruce N.

Kitchell, both of Williamsville, N.Y.; [57] ABSTRACT Amomo Suva SanPaulo Bram Mill spring constant change signals are produced after [73]Assignee: Westinghouse Electric Corporation, each of a series ofscrewdown regulation cycles where Pitt b r h, Pa, each change signal isproduced as a function of the ratio of the gauge deviation remainingafter the regu- [221 July 1972 lation cycle, to the change in rollingforce which oc- [21] App]. No.1 273,896 curred during the regulationcycle.'The series of ratio change signals are then averaged to produce amill spring constant correction signal which is used to cor- [ii] metthe previously established mi" Spring constant 72 8 G The corrected millspring constant is then used to gen- 1e 0 care l7; 1 erate a newScrewdown reference signal for the next I 6 series of regulation cycles.Provision is made to limit check and average gauge deviation so that themill [56] keferencesclted spring corrections are only made when certainprede- UNITED STATES PATENTS termlned gauge deviation l1m1ts areexceeded.

3,253,438 5/1966 Stringer 72/12 3,625,037 12 1971 Michel 12/21 x 18Chums, 9 Drawing; Flgul'es I ZE DP 50o JEQUENCE Bili'iEii-Ll CONTROLsogaoczssoa .00 l stream 1 SCREWDOWN REF \SCREWDQWN 1 POSITION HLFERENCESIGNAL 5H0 REGULATOR PROCESSOR DKMAVG XTEMP SA F :1 SA U DEVIATION 300 I1 'IIAi'III. LIMIT TESTING g GAVERAGiNG I22 PROCESSOR .7 X/o XM 1/POSITION UtltClUH 1 '50 1212 DESIREDGAUGE I t I [I23 '3' SIBUULRSEE LTRD PATENTEBHEB! 1 ms SHEET 2 [If 3 mmQE METHOD AND APPARATUS FOR COLDROLLING MILL GAUGE DEVIATION CORRECTION BACKGROUND OF THE INVENTIONRolling mill control systems are presently available wherein a screwdownposition regulator is employed to drive a motor to establish a screwdownposition corresponding to a reference input signal. The reference inputsignal has, heretofore, been developed as a function of the summation ofan initial screwdown setting, a gauge deviation measurement, and a millspring corrction factor introduced to compensate for gauge deviation dueto mill stretch. In the present practice, the mill spring correctionfactor is established as the product of a mill spring constant andmeasured rolling force change.

The mill spring constant (referred to as KM herein) has, heretofore,been set for an average product withmanual variations being made toaccount for changes in the width of the strip to be rolled. It has beenfound, however, that temperature changes and other factors cause theso-called KM constant to vary during rolling so that the screwdownposition regulator cannot establish the proper screwdown position toproperly eliminate the measured gauge deviation. As a result, someproduct is lost before the system operation can be adjusted to make theproper correction to reduce the gauge deviation to less than anacceptable limit.

SUMMARY OF THE INVENTION A method and apparatus forautomaticallyupdating the mill spring constant (KM) during rolling andthen for continuously testing gauge deviations to adjust this constantare provided. The screwdown position is regulated, as previously, toreduce the error between the screwdown reference (REF) and the actualmeasured screwdown position (SA) to substantially zero. When thisapproximate zero is detected, a transport delay cycle is initiatedduring which roll movement pulses are counted to precisely measure thedistance between the rolling stand and the point of gauge deviationmeasurement. The gauge deviation remaining from the previous regulationcycle is then divided by the rolling force change which occurred duringthe regulation cycle to produce, a first mill spring constant change ordifference signal (DKM). The regulation cycle is then repeated as soonas another gauge deviation is measured which exceeds a predeterminedlimit and, when the approximate zero error signal is detected during thesecond regulation cycle a second transport delay count is performed andthen the second DKM signal is generated and added to the first toproduce a SUMDKM DKMI DKM2. The above cycles are repeated providing anumber (N) of DKM signals and SUMS thereof so that an average millspring correction signal DKMAVG may be developed as:

DKMAVG SUMDKM/N.

and improves the gaugeregulationaccuracy. It is estimated that anincrease from percent to approximately percent of the bar or ingotrolled may be kept within the desired gauge limits with the method ofthe invention.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which from a part of this specifica tion,and in which:

FIG. l is a block diagram of a system employing the present invention;

FIG. 2 is a flow diagram setting forth the function of sequence control200 of FIG, 1;

FIGS. 3A and 3B are functional flow diagrams setting forth the functionof processor 300 of FIG. 1;

FIGS. 4A and 4B are flow diagrams setting forth the function ofscrewdown reference signal processor 400 as it is associated withscrewdown position regulator FIG. 5 is a flow diagram setting forth thefunction of transport distance processor 500; and

FIGS. 6A and 6B are flow diagrams setting forth the function of millspring averaging processor 600 of FIG.

In FIG. 1, thescrewdown position of the rolls in a mill stand 120 iscontrolled through a motor which receives an appropriate drive signalfrom a screwdown position regulator 100. Mill stand includes appropriatescrewdown means 121 which includes a load cell or other device forproducing a rolling force signal F which is used in the processingfunction of means 400 to be described. The stand also includes ascrewdown position measuring device 122 which produces a signal SAcorresponding to the actual screwdown position therein. The function ofregulator 100 is to drive screwdown mechanism I21 until the differencebetween signals REF and SA is approximately zero. While the regulatormay be conventional in most respects it includes additional means fordetecting the approximate zero condition in the difference betweensignals REF and SA and for producing an output signal ZE representingthe occurrence of the approximate zero error.

A sequence control 200 is provided which may be part of a computerprogram which will be described which responds to the zero error signalZE and initiates a transport distance counting cycle in a processor 500which also receives roll movement pulses T]? from a suitable transducer123 in stand 120. The function of processor 500 is to count signals TPuntil strip being rolled has moved through the distance referenced asTRD between the rolls and an XRAY measuring station where an X-raysource 131 provides XRAYs which are read by a gauge deviation measuringdevice 132.

Device 132 produces a gauge deviation output signal referenced as X% anda gauge deviation measurement signal XM which are applied to a deviationlimit testing and averaging processor 300 which will be considered indetail with respect to FIGS. 3A and 3B. The function of processor 300 isto detect gauge deviations exceeding a predetermined limit such as 1%and to average successive such deviations to produce a signal referencedas XAVG which is applied to processor 400.

When the transport distance TRD has been moved following the completionof a zero regulation cycle, processor 500 produces a signal referencedas DP which initiates the operation of mill spring constant averagingprocessor 600. The function of processor 600 at this time is to computethe ratio DKM KTEMP/DF where XTEMP is the gauge deviation measurement XMmade immediately following the zero error detection and DF is the changein rolling force which occurred during the last regulation cycle beforethe zero error. Processor 600 includes means for summing successive DKMsignals until a predetermined number N has been generated and then isoperated to divide by that number to produce an average mill springcorrection signal DKMAVG which may be defined as SUMDKM/N, where SUMDKMrepresents the summation of N successive DKM signals as defined above.After appropriate limit tests are made upon signal DKMAVG to determinewhether the correction is acceptable, the previous mill spring constantKM is corrected by adding the average correction if it is withinacceptable limits. If not, as will be described below, a suitable alarmis generated.

While the various processors and sequence control of FIG. 1 may beprovided through the use of special purpose analog or digital circuitsor combinations thereof, it is preferred in the practice of theinvention to utilize a programmed computer to accomplish the variousfunctions since the computer may also be used to perform other controlfunctions not associated with the invention. It will be understood,therefore, that the term processor is employed to represent that circuitoperation of the programmed computer which performs the function definedby the program described herein. In practice, the program may be readfrom a wired or read-only memory or be entered as software into ageneral purpose computer. In some cases, as in the case of the transportdistance processor, a combination of hard-wired circuits and a computerprogram may be employed to accomplish the desired function. Beforeconsidering the specific details of the computer program foraccomplishing the function set forth in FIG. 1, the method of controlwill be considered. The refer- 'ence signal applied to regulator 100 maybe defined as:

REF SO XAVG KM(DF) where DF FA F0.

The term SO represents the initial screwdown position measured by signalSA at the beginning of a regulation cycle, XAVG represents the averagedgauge deviation measurement preceding the initiation of the regulationcycle and DF, representing the rolling force change, is initially set tozero at the beginning of the regulation cycle with FA FO (initialrolling force). Regulator 100 is then operated in a well-known manner toreduce the error signal ERR REF SA to substantially zero while processor400 continuously updates signal REF as the rolling force changes so thatDF has a value. When this has occurred, as is noted in FIG. 4B, a zeroerror flag (ZE) is set in the computer (providing the functions ofFIG. 1) and is detected in the operation of the sequence control in FIG.2 (step 209) to initiate transport distance processing, the function ofwhich is set forth in FIG. 5. Many methods of carrying out the transportdistance counting will be apparent to those skilled in the art. Onesuitable method of operation is to utilize a separate counting circuitwhich receives'a comparison input signal corresponding to distance TRDand counts the roll pulses TP until the number of pulses counted isequal to TRD.

In the operation of regulator 100, the screwdown reference signal REF iscontinuously changed by the function KM(DF) because the rolling forcevaries as the screwdown position is changed. Thus, at the end of theregulation cycle, the difference DF FA PC has been measured and isavailable for use as soon as the transport distance processing has beencompleted. Thus, it will be seen in the specific flow diagram of FIG. 2,that a test is made in sequence control to determine if signal DP hasbeen set; and if so, the control proceeds to calculate DKM XTEMP/DF aspreviously noted. Following this calculation, deviation limit testingand averaging is performed, as will be considered in further detailbelow, in order to detect gauge deviations and as soon as two successivedeviations exceed a limit in the preferred program shown herein, a newinitial reference signal REF is generated where the rolling forcedifference signal is again set to zero and a new regulation cyclebegins.

It may be considered that sequence control 200 includes a counter whichdetermines the number of times the change signal DKM is produced andsummed in processor 600 or that this is part of the program of processor600. After a number of summations of signals DKM have been made (in FIG.6A six are made), processor 600 is caused, by program control to dividethe total summation by N (6) to produce the average signal DKMAVG whichis added to the previous mill spring constant KM and the entire controlprocess is reinitiated.

The flow diagram in FIG. 2 generally sets forth the sequence controlsteps previously noted. At the start of the sequence, a test is made instep 201 to determine whether an XRAY reading has just been made. Ifthis has occurred, this reading is stored in a computer memory locationreferenced as XTEMP in step 202 and a test is then made in step 203 todetermine whether the flag DP previously noted has been set. Flag DP isset, it will be recalled, if the transport measurement has beencompleted. If not, the sequence control terminates and reinitiates ifthere is an XRAY reading or, if not, if calculation is to be performedfor regulation. Computer means for reinitiating sequencing cycles arewell known and will not be described. Basically, such means respond tointerrupts or other program initiation signals. It may be assumed thateach time an XRAY reading is made an interrupt causes the start whereasthe calculation of the reference updating for the purpose of regulationmay be made on a periodic basis. For the purpose of explanation, it willbe assumed that flag DP has been set and the next test which must beperformed is to determine whether reading XTEMP relates to the firstdeviation after the transport distance TRD has been measured or not.This is performed in step 204. Step 204 is based upon a flag referencedas KMCK which is initially reset and then is set in step 315 in theprogram of FIG. 33. For the purpose of explanation, it will be assumedthat flag KMCK is not ON (exit N) and that program control then goes toprocessor 3000 (TLIM), the function of which will be described withreference to FIG. 3A. This program is referenced as TLIM to summarizeits function of limiting and testing gauge deviation.

Referring now to FIG. 3A, it is noted that the first test (step 301) iswhether XTEMP is greater than a +1 percent deviation. If it is, step 302tests to see if a flag referencedas PLIM has been set. If so, theprogram of FIG. 3B(XAVG) is entered where the gauge deviation isaveraged. If not, flag PLIM is set in step 303 and then signal XTEMP isstored as XRAYI in step 305 and the sequence ends. If XTEMP does notexceed a positive 1 percent deviation, a test is made in step 306 to seeif it exceeds a negative 1 percent deviation. If so, a test is madetodetermine if the NLIM flaghas been set in step 307. If not, this flag isset in step 304 and step 305, previously noted, is performed and thesequence is ended. If theNLIM flag had been set as noted in step 307,then exit is made to the XAVG function described in FIG. 3B. If step 306does not detect a negative deviation exceeding 1 percent, thensteps 308and 309 are entered where both flags PLIM and NLIM are reset and thesequence is ended.

The function set forth in FIG. 3A is to first detect one occurrence of a1 percent deviation either plus or minus and set an appropriate flag andthen, after the next XRAY reading, to determine whether a seconddeviation of the samesense has occurred. If two successive deviationsexceeding a gauge deviation limit have occurred, then the XRAY averagingoperation described in FIG. 3B is entered. If a single deviationoccursfollowed by the failure of the deviation to exceed either limit,both of the flags are reset to reinitiate the testing.

In FIG. 313, step 310 provides for the averaging of XRAYl and XTEMP bysumming them and dividing by 2. In step 311, a roll force reference flagR FRFG is set and is tested in step 205 of FIG. 2 as will be discussedlater. The purpose of flag RFRFG is to signal to control 200 that theinitial rollforce F must be stored before beginning the next regulationcycle. Step 312 in FIG. 3B callsfor the reading of screwdown signal SAto establish the initial screwdown signal SO used by processor 400. Instep 313, the roll force difference signal DF is set to zero and thenthe zero error flag ZE is reset in step 314. Following this, the firstdeviation cycle flag KMCK is set in step 315 and than in step 316, flagDP is reset. This completes the steps which are necessary to prepare thesystem for processing a new reference signal (REF) as is done in step401 of FIG. 4A.

Before considering the reference signal processing and regulation,reference is made again to FIG. 2 where now it will be, considered thatflag KMCK has been set and step 204 of FIG. 2 then causes the system toproceed to first reset flag KMCK in step 206 and then to proceed to theDKM computation program of FIG.6A. In step 601 of FIG. 6A, thecalculation DKM XTEMP/DP is made and this is followed by the summationstep 602 where DKM is added to. the previous SUMDKM. It may be assumedthat the system is initialized with SUMDKM equal to zero and this sum isreset to zero in step 610 of FIG. 6B to reinitiate another averagingcycle. In step 603, an increment I is increased which may be assumed tobe initially equal to one as it is set in step 611 of FIG. 6B, and thisis followed by a test in step 64 to determine whether the desired numberof summations has been performed. As an example, the test is madeagainst seven which would provide six summations before completing thissequence. If the increment I is equal to seven the program proceeds tothe DKMAVG function of FIG. 6B which will be briefly described. in step605, DKMAVG is generated as SUMDKM/6 and then, in steps 606 and 607,tests are made to determine whether the averaged correction exceeds plusor minus 0.2KM. If these tests reveal that DKMAVG is within 0. IKM, anew mill spring constant KM is generated equal to the summation of theprevious signal KM and the correction IDKMAVG in step 609. If either oftests 606 or 607 reveals too large a correction, an alarm is set in step608. The sequence of FIG. 6B then proceeds to reset the SUMDKM in step610 previously noted and to reset the increment I to l in step 611.

Each time the deviation limit testing and averaging function ofprocessor 300 has been completed, a new reference is generated as setforth in step 501 of FIG. 4A where signal S0 is the initial screwdownsetting read during step 312 of FIG. 3B and the roll difference signalDF has been set to zero in step 313 in FIG. 313. Step 402 is consideredto be a call for the screwdown position regulator program which isbrought into operation through well known-programming means and isgenerally summarized in FIG. 4B. As previously noted,

the function of regulator is to control motor until the position errornoted in step 403 is substantially zero. If the error tested in step 404is not greater than zero step 405 provides for the setting of the flagZE which is used in the sequence control to initiate the transportdistance measurement as previously noted. If the error is greater thanzero step 406 is entered to perform a function translation which must beperformed to translate the position error (REF SA) into a speed controlfor motor E10. This may be performed as a table look-up function withina programmed computer or in a well known analog function generator.

Referring now again to FIG. 2, it will be assumed that the regulationcycle is in process and that an XRAY reading is not being made. The rollforce flag has been set in step 311 of FIG. 3B and therefore in step207, the initial roll force F0 is measured and stored. Following this,the flag RFRFG is reset in step 203 and the sequence is ended. When thesequence reinitiates, step 205 causes proceeding to step 209 where thezero error flag ZE is tested and will be assumed, for the present, tonot have yet been set. Therefore, the program enters the referencesignal generating operation (REF), described above, with reference toFIG. dA, to change the screwdown setting until the zero error flag isfinally set after several sequences of this type in step 405 of FIG. 4B.When this is detected in step 209, the system enters the transportdistance measuring sequence of FIG. 5 which will not be described.

In step 501, a roll pulse TP is counted and then in step 502, the countis compared to the representation of the transport distance TRD. if thecount is equal to TRD, flag DP is set in step 503 which prepares thesystem, as previously noted, to enter deviation testing and DKMcomputation. If the count is not equal to TRD the function of step 501is repeated. The flow diagram of FIG. 5 does not represent an actualprogram since the counting and comparison function may preferably. beperformed in a special circuit which generates an interrupt for thecomputer system when the pulse count is equal to TRD. Step 503,therefore, may be considered to be part of an interrupt program whichgoes into operation as soon as the pulse count is equal to TIRD and setsflag DP.

From the foregoing description it should now be apparent that thepresent invention provides an effective means for providing mill springconstant corrections on-line during mill rolling where the correction isaveraged over a number of cycles and appropriately limited. While thecontrol has been described as a general purpose computer program, itwill be understood that special purpose digital systems may beconstructed to perform the functions of the program and in some casescertain functions may be performed with analog components to providewhat is referred to, in the art, as a hybrid system.

We claims as our invention:

1. The method of rolling a strip of ductile material to produce apredetermined output gauge comprising measuring gauge deviationsremaining after each of a series of screwdown regulation cycles;producing a corresponding series of ratio signals after each occurrenceof a zero error signal in a preceding regulation cycle, where each ratiosignal is produced by dividing the remaining gauge deviation by thechange of roll force which occurred during the preceding regulationcycle; and averaging the ratio signals to produce a mill springcorrection signal for the next series of regulation cycles.

2. The method of claim 1 wherein each ratio signal is generated afterthe strip has moved from the point of rolling to the point of deviationmeasurement spaced from said point of rolling by a predeterminedtransport distance.

3. The method of claim 1 wherein the regulation cycle is controlledaccording to a reference signal produced as a function of the initialscrewdown position at the start of a regulation cycle, the gaugedeviation existing at the beginning of the cycle, and the product ofmill spring constant times the change of roll force necessary to reducethe error signal to zero.

4. The method of claim 3 wherein said reference signal is defined as REFSO XAVG KM(FA F) where S0 is the initial screwdown position; XAVG is anaveraged XRAY deviation signal; KM is the last corrected mill springconstant; F0 is the initial rolling force and FA is the measured rollingforce during the screwdown regulation cycle.

5. The method of claim 4 wherein said ratio signal is represented by thefunction DKM XO/DF where DKM represents a change in mill spring constantthat is to be electrically averaged, X0 is the gauge deviation remainingafter the last regulation cycle and DF represents the change of rollforce which occurred during the last regulation cycle.

6. The method of claim 3 wherein each regulation cycle is continueduntil REF SA 0, where SA represents the actual screwdown position.

7. The method of claim 5 wherein said averaging step is performed byperforming the summation of signal DKM a number of times and then bydividing the final summation by the number to produce said mill springconstant correction signal.

8. In a system for automatically controlling the gauge of a strip ofmaterial rolled through a mill stand, where a position regulatorreceives a screwdown reference signal and a signal representing theactual screwdown position and responds thereto to adjust the rollingforce until the error between the reference signal and the positionsignal is substantially zero, a gauge deviation correction controlcomprising: first means for measuring the gauge deviation at apredetermined transport distance from the rolling stand; second meansoperative each time after said position regulator has produced asubstantially zero error signal and after the strip element then hasmoved through said predetermined transport distance to said second meansfor producing a mill spring constant correction signal proportional tothe average summation of gauge deviation divided by roll force change.

9. The gauge deviation correction control of claim 8 wherein saidposition regulator is controlled by a reference signal produced as afunction of initial screwdown position, average XRAY deviation exceedinga predetermined limit and the product of the last set of mill springconstant times change in roll force.

10. The gauge deviation correction control defined in claim 9 whereinsaid reference signal is defined by the function REF SO XAVG KM(FA F0).

11. The correction control of claim 10 wherein each change signal isdefined as DKM XO/DF where XO represents gauge deviation remaining afterzero error regulation and DF is the change of rolling force whichoccurred during the zero regulation cycle.

12. The gauge deviation control of claim 11 wherein said second meansproduced an average correction signal DKMAVG SUMDKM/N where SUMDKMrepresents the summation of a series of N change signals DKM.

13. In combination: a screwdown position regulator responsive to areference signal and a screwdown position signal for adjusting therolling force until the error between the reference signal and theposition signal is substantially zero; deviation limit testing andaveraging means for producing a gauge deviation signal representingaverage gauge exceeding predetermined limits; a mill spring averagingprocessor for producing an averaged mill spring correction signal as afunction of a number of gauge deviations remaining after the operationof said position regulator and as an inverse function of thecorresponding rolling force changes occurring therein; and a screwdownreference signal processor for periodically updating the referencesignal as a function of said averaged mill spring correction signal.

14. The combination of claim 13 wherein a transport distance processoris included for precisely determining when an element of rolled stripcorresponding to zero regulation error has moved from the rollingposition to the position of deviation measurement.

15. The combination of claim 13 wherein a sequence control is providedto cause operation of said deviation limit test and averaging such thattwo successive deviations must be detected to exceed a predeterminedlimit before the next regulation control is caused to begin.

16. The combination of claim 15 wherein said predetermined limits areand 1 percent.

17. The combination of claim 13 wherein said screwdown reference signalprocessor produces said reference signal according to the function REF$0 XAVG KM(FA F0).

18. The combination of claim 13 wherein said mill spring averagingprocessor produces said averaged correction signal as the functionDKMAVG SUMDKM/N where SUMDKM represents the summation of a series of Nmill spring changes.

1. The method of rolling a strip of ductile material to produce apredetermined output gauge comprising measuring gauge deviationsremaining after each of a series of screwdown regulation cycles;producing a corresponding series of ratio signals after each occurrenceof a zero error signal in a preceding regulation cycle, where each ratiosignal is produced by dividing the remaining gauge deviation by thechange of roll force which occurred during the preceding regulationcycle; and averaging the ratio signals to produce a mill springcorrection signal for the next series of regulation cycles.
 2. Themethod of claim 1 wherein each ratio signal is generated after the striphas moved from the point of rolling to the point of deviationmeasurement spaced from said point of rolling by a predeterminedtransport distance.
 3. The method of claim 1 wherein the regulationcycle is controlled according to a reference signal produced as afunction of the initial screwdown position at the start of a regulationcycle, the gauge deviation existing at the beginning of the cycle, andthe product of mill spring constant times the change of roll forcenecessary to reduce the error signal to zero.
 4. The method of claim 3wherein said reference signal is defined as REF SO + XAVG + KM(FA - FO)where SO is the initial screwdown position; XAVG is an averaged XRAYdeviation signal; KM is the last corrected mill spring constant; FO istHe initial rolling force and FA is the measured rolling force duringthe screwdown regulation cycle.
 5. The method of claim 4 wherein saidratio signal is represented by the function DKM XO/DF where DKMrepresents a change in mill spring constant that is to be electricallyaveraged, XO is the gauge deviation remaining after the last regulationcycle and DF represents the change of roll force which occurred duringthe last regulation cycle.
 6. The method of claim 3 wherein eachregulation cycle is continued until REF - SA 0, where SA represents theactual screwdown position.
 7. The method of claim 5 wherein saidaveraging step is performed by performing the summation of signal DKM anumber of times and then by dividing the final summation by the numberto produce said mill spring constant correction signal.
 8. In a systemfor automatically controlling the gauge of a strip of material rolledthrough a mill stand, where a position regulator receives a screwdownreference signal and a signal representing the actual screwdown positionand responds thereto to adjust the rolling force until the error betweenthe reference signal and the position signal is substantially zero, agauge deviation correction control comprising: first means for measuringthe gauge deviation at a predetermined transport distance from therolling stand; second means operative each time after said positionregulator has produced a substantially zero error signal and after thestrip element then has moved through said predetermined transportdistance to said second means for producing a mill spring constantcorrection signal proportional to the average summation of gaugedeviation divided by roll force change.
 9. The gauge deviationcorrection control of claim 8 wherein said position regulator iscontrolled by a reference signal produced as a function of initialscrewdown position, average XRAY deviation exceeding a predeterminedlimit and the product of the last set of mill spring constant timeschange in roll force.
 10. The gauge deviation correction control definedin claim 9 wherein said reference signal is defined by the function REFSO + XAVG + KM(FA - FO).
 11. The correction control of claim 10 whereineach change signal is defined as DKM XO/DF where XO represents gaugedeviation remaining after zero error regulation and DF is the change ofrolling force which occurred during the zero regulation cycle.
 12. Thegauge deviation control of claim 11 wherein said second means producedan average correction signal DKMAVG SUMDKM/N where SUMDKM represents thesummation of a series of N change signals DKM.
 13. In combination: ascrewdown position regulator responsive to a reference signal and ascrewdown position signal for adjusting the rolling force until theerror between the reference signal and the position signal issubstantially zero; deviation limit testing and averaging means forproducing a gauge deviation signal representing average gauge exceedingpredetermined limits; a mill spring averaging processor for producing anaveraged mill spring correction signal as a function of a number ofgauge deviations remaining after the operation of said positionregulator and as an inverse function of the corresponding rolling forcechanges occurring therein; and a screwdown reference signal processorfor periodically updating the reference signal as a function of saidaveraged mill spring correction signal.
 14. The combination of claim 13wherein a transport distance processor is included for preciselydetermining when an element of rolled strip corresponding to zeroregulation error has moved from the rolling position to the position ofdeviation measurement.
 15. The combination of claim 13 wherein asequence control is provided to cause operation of said deviation limittest and averaging such that two successive deviations must be detectedto exceed a predetermined limit before the next regulation control iscaused to begin.
 16. The combination of claim 15 wherein saidpredetermined limits are + and - 1 percent.
 17. The combination of claim13 wherein said screwdown reference signal processor produces saidreference signal according to the function REF SO + XAVG + KM(FA - FO).18. The combination of claim 13 wherein said mill spring averagingprocessor produces said averaged correction signal as the functionDKMAVG SUMDKM/N where SUMDKM represents the summation of a series of Nmill spring changes.